CO.FLAP.SH5(1).Mount Evans Highway. Geotech Rpt FINAL.September 2023_Air Convection Embankment (002).pdf

MOUNT EVANS HIGHWAY

CLEAR CREEK COUNTY, CO

CO FLAP SH5(1)

FINAL Geotechnical Report

Prepared by

Federal Highway Administration Central Federal Lands Highway Division

Geotechnical Services Branch

September 2023 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

Brendan L. McGarity, E.I., Geotechnical Engineer

Report reviewed by: ____________________________________________________________

James M. Arthurs, P.E., Ph.D., Senior Geotechnical Engineer

Approved for distribution by: ____________________________________________________

Devin T. Dixon, P.E., Lead Geotechnical Engineer

Distribution

N:\CO\SH5(1)\Geotech\7_FinalDocs

K:\TechServices\Geotech\1. Geotech Project Reports

Project Management

Project Development, Lead Designer

Attachments

CO FLAP SH5(1).Mt Evans.Geotechnical Preliminary Investigation Notes.2020.kmz

CO FLAP SH5(1).Mt Evans.Geotechnical Preliminary Investigation Photos.2020.zip

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TABLE OF CONTENTS

SECTION ONE - INTRODUCTION

1.1 Background and Location

1.2 Scope and Purpose

SECTION TWO - GEOLOGY AND SEISMICITY

2.1 Regional Geology

2.2 Site Geology

2.3 Regional Seismicity

2.4 Seismic Design Parameters

2.5 Geologic Hazards

SECTION THREE - SUBSURFACE INVESTIGATION

3.1 Subsurface Exploration Program

3.2 Laboratory Testing Program

3.3 Summary of Site Conditions

3.3.1 General Topography

3.3.2 Surface Reconnaissance

3.3.3 Subsurface Conditions

3.3.4 Groundwater

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 Air Convection Embankment Design

4.1.1 Design Selection

4.1.2 Thermal Modeling

4.1.3 Air Convection Embankment Recommendations

4.2 Earthworks

4.2.1 Shrink/Swell Recommendations

4.2.2 Cut Slopes

4.2.3 Embankments

4.2.4 Granular Section

4.3 Construction Considerations

4.4 Post Construction Considerations

SECTION FIVE - DISCLAIMER/LIMITATIONS CLAUSE

SECTION SIX - REFERENCES

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TABLES

Table 2.1:- Summary of Nearby Faults Table 2.2:- Summary of Seismic Parameters Corrected for Site Class C Table 3.1:- Summary of the Field Exploration Program

Table 3.2:- Summary of Laboratory Index Test Results Table 3.3:- Summary of Laboratory Engineering Property Test Results Table 3.4:- Summary of Permafrost and Groundwater Depths Encountered During Drilling

Table 4.1:- Summary of Model n-factors

Table 4.2:- Summary of AIR/W Parameters Table 4.3:- Summary of TEMP/W Parameters Table 4.4:- Air Convection Embankment Model Results

FIGURES

Figure 1.1: Typical cracking and existing conditions on SH5

Figure 1.2: Existing dips and ridges in pavement Figure 4.1: Model geometry and material regions

Figure 4.2: Model mesh for embankment and foundation soils Figure 4.3: Results of spin-up analysis, 12/31/2021) Figure 4.4: Results of no ACE construction projected to 12/31/2050

Figure 4.5: 4-foot ACE with 1V:2H side slopes. Output from preliminary analysis Figure 4.6: 4-foot ACE with 1V:6H side slopes. Output from preliminary analysis

Figure 4.7: 4-foot ACE with 1V:2H side slopes, 12/31/2050. Zoomed Figure 4.8: 6-foot ACE with 1V:2H side slopes, 12/31/2050. Zoomed Figure 4.9: 4-foot ACE with 1V:2H side slopes, 12/31/2050

Figure 4.10: 6-foot ACE with 1V:2H side slopes, 12/31/2050

Figure 4.11:- Modeled permafrost depths from construction to 2050

PLATES

Plate 1:- Project Vicinity Map

Plate 2:- Geology Map

Plate 3:- Geology Map Legend

Plate 4:- Geotechnical Boring Locations

APPENDICES

APPENDIX A - Field Exploration Program

APPENDIX B - Laboratory Test Results

APPENDIX C - Photographs

APPENDIX D - Excavation Characteristics of Rock & Rippability Charts file://///cflcadd4.cflhd.local/projects/CO/SH5(1)/Geotech/7_FinalDocs/CO.FLAP.SH5(1).Mount%20Evans%20Highway.%20Geotech%20Rpt%20FINAL.August%202023_Air%20Convection%20Embankment.docx%23_Toc141705357 file://///cflcadd4.cflhd.local/projects/CO/SH5(1)/Geotech/7_FinalDocs/CO.FLAP.SH5(1).Mount%20Evans%20Highway.%20Geotech%20Rpt%20FINAL.August%202023_Air%20Convection%20Embankment.docx%23_Toc141705358

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Mount Evans Highway CO FLAP SH5(1) September 2023

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering assessment and provides recommendations for roadway reconstruction to be performed on Scenic Byway State Highway 5

(Mt. Evans Highway) in Clear Creek County, Colorado. The project is located within the Mount

Evans Wilderness, roughly 8.5 miles south of the intersection of SH103 and SH5. The project limits cover about 0.7 miles of roadway south of the lower Summit Lake Trailhead gravel parking area. SH5 is the highest paved road in the United States at an elevation of 14,130 feet above sea level. At the project location, the elevation is approximately 13,000 feet. The weather at this elevation is extremely harsh, creating a difficult environment for a paved road. Mount Blue Sky

(formerly Mount Evans) receives on average 292 inches of snowfall per year with temperatures ranging from a high of 65 degrees to a low of -40 degrees Fahrenheit. A site map is presented on

Plate 1 of this report.

The project roadway provides access to hiking, climbing, cycling, scenic driving, and nature viewing. The current roadway is extremely distressed with major subgrade failures. The roadway was originally constructed across a wetland complex causing disruption to both surface and subsurface water flow, leading to degradation of the underlying permafrost. Permafrost degradation and on-going solifluction have caused major damage to the roadway including settlement, subgrade failure, and cracking (Figures 1.1 and 1.2).

1.2 SCOPE AND PURPOSE

The purpose of this project is to correct subgrade issues, improve drainage, and restore hydrological function. The roadway section is to undergo major reconstruction and be supported upon an air convection embankment (ACE) to improve subsurface drainage, minimize permafrost degradation, and stabilize the roadway. Roadway work is also anticipated to include both removal and in-kind replacement of existing culverts, rebuilding of the subgrade, and repaving.

The scope of work included a geotechnical investigation, analysis, and recommendations for roadway embankment for use in design and construction. This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, interpretation and correlation of field measurements, and geotechnical engineering modeling and analysis. Specifically, this investigation was conducted to determine the soil profiles along the project alignment and develop recommendations concerning roadway embankment, subsurface drainage, geologic hazards, and construction considerations for the design and construction of the roadway within the alignment.

The stationing in this report is based on the in-progress preliminary 95% project plans.

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Figure 1.1: Typical cracking and existing conditions on SH5.

Figure 1.2: Existing dips and ridges in pavement.

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SECTION TWO - Geology and Seismicity

2.1 REGIONAL GEOLOGY

The project is located in the Front Range of the Southern Rocky Mountains physiographic province. The region is typically underlain by Proterozoic granodiorite of the Mount Evans batholith (YdgM), which formed as a large, deep intrusion of magma approximately 1.4 billion years ago (Kellogg et al., 2008). Since the initial intrusion and uplift, the region experienced long periods of erosion and two major advances and withdrawals of a broad seaway engulfing the area

(USGS, 1967). The most recent stage of uplift, the Laramide Orogeny, approximately 70 million years ago, largely created the Rocky Mountains as we see them today.

During the Pleistocene Epoch, large masses of snow and ice accumulated high in the mountains and moved down the valleys as glaciers (USGS, 1967). The high-altitude landscape is marked with cirques, U-shaped valleys, moraines, tarns, and other glacial features. After the retreat of the glaciers, about 6,500 years ago, erosion continued largely through the downcutting of streams into the mountains and foothills.

2.2 SITE GEOLOGY

Geologic mapping of the project area depicts SH5 as underlain by Quaternary Colluvium (Qc) which is underlain by Granodiorite of the Mt. Evans batholith (YdgM) (Kellogg et al., 2008).

Quaternary talus (Qt) and rock glacier deposits (Qrg) are also mapped in the project vicinity. The

Quaternary Colluvium is likely composed of the weathering products of the underlying granodiorite and was likely formed during Pleistocene to recent glacial activity, as well as modern erosion. Colluvium include particles ranging from clay to boulder sized.

Refer to Plates 2 and 3 to view the geologic map and further unit descriptions that correspond to the project site.

2.3 REGIONAL SEISMICITY

Two seismic source faults are mapped within 50 miles of the project area and are summarized below in Table 2.1. Both seismic source faults are located west of the project and exhibit slip rates of less than 0.0079 inches per year, with the most recent deformation taking place in the last 15,000 years.

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Table 2.1:- Summary of Nearby Faults

FAULT OR

FAULT ZONE

DISTANCE

FROM CENTER

OF PROJECT

FAULT

PARALLEL

SLIP RATE

FAULT

LENGTH

TIME OF MOST

RECENT

DEFORMATION

MAXIMUM

PROBABLE

MAGNITUDE

(miles) (inch/year) (miles) (years)

Gore Range

Frontal Fault

25.6 <0.0079 46.6 <15,000 7.00

Williams Fork

Mountains Fault

32.8 <0.0079 23.6 <15,000 6.54

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters are based on the American Association of State

Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 8th

Edition, 2017, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1000-year return period). The 1000-year return period uniform hazard spectrum for SH5 (39.597656º N latitude and 105.640650º W longitude) was obtained in accordance with the AASHTO ground motion maps.

The site was classified as “Class C” according to site class definitions specified in Table 3.10.3.1-

1 of AASHTO based on a very dense soil and soil rock profile. The recommended spectral acceleration coefficient values for probabilistic design are summarized in Table 2.2.

Table 2.2:- Summary of Seismic Parameters Corrected for Site Class C

Peak Seismic Ground Acceleration Coefficient, (As) 0.084

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.175

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.064

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.2

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 1.2

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.7

Based on the long acceleration coefficient SD1 value of 0.064, the site is assigned to seismic hazard

Zone 1 in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table

4.7.4.3.1-1 in AASHTO. Based on this classification, seismic loading is assumed to not control design of any improvements for this project.

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2.5 GEOLOGIC HAZARDS

The project site is located in a mountainous, high alpine environment overlying colluvium with a zone of permanently frozen ground (permafrost). Accelerated thawing of ice-rich permafrost poses significant geotechnical hazards to roads. Thawing reduces both the bearing capacity and compressive strength of frozen soils and the increased pore water pressure from melt water reduces slope stability. This produces settlement and slope instability in unconsolidated sediments, and rockslides and debris flows in more competent lithologies. For roads built on-grade or with low embankments, like SH5, longitudinal subsidence beneath the centerline can create a concave road cross section. Extreme localized deformation also occurs through boulder jacking from the subgrade. Rapid settlement beneath embankment slopes and toes is also common and causes shoulder rotation and longitudinal cracking along the centerline (Shaw & Cooper, 2021).

In addition to permafrost concerns, portions of the roadway were constructed over wetland and fen areas. It is likely that soils with high organic contents are present under these portions of the alignment. Organic soils are commonly highly porous and compressible, likely further contributing to increased settlement and poor pavement performance.

These hazards discussed above can be mitigated by appropriate embankment, subgrade, and drainage design. Additionally, to ensure long term function and lifespan of the roadway, proper, consistent maintenance of the proposed embankment structure is vital. The porous, open nature of the embankment allows surface water to diffuse through the embankment and air to circulate within it, allowing the natural hydrology to largely return to its natural state, and the underlying soils to remain relatively cool from air flow. The roadway owner, to the greatest extent possible, should not push dirt or other debris onto the embankment side slopes to ensure the open nature of the structure is maintained.

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SECTION THREE - Subsurface Investigation

3.1 SUBSURFACE EXPLORATION PROGRAM

A subsurface investigation was performed by a Central Federal Lands Highway Division

(CFLHD) of the Federal Highway Administration (FHWA) geotechnical and pavements engineers on September 17th through the 18th, 2020. The geotechnical subsurface exploration program consisted of drilling a total of seven (7) borings ranging from 20 to 30 feet in depth. Borings were advanced utilizing ODEX (Overburden drilling EXcentric) drilling. Standard penetration testing

(SPT) with both SPT and brass California liner sample collection was performed at 5-foot intervals in each boring, with an additional SPT at a shallow 2.5-foot depth. Subsurface conditions were logged, and representative samples were collected and transported to the CFLHD Materials

Laboratory in Lakewood, CO, for physical property testing. Logs of the explorations from the pavements engineer and boring locations are presented in Appendix A and Plate 4, respectively.

Photographs related to this exploration may be found in the final Pavement Report. A summary of the field exploration is provided in Table 3.1.

Table 3.1:- Summary of the Field Exploration Program

EXPLORATION

DESIGNATION

LANE STATION OFFSET

GROUND

ELEVATION

(ft)

TERMINATION

ELEVATION (ft)

EXPLORATION

DEPTH (ft)

BH20-02 Southbound 29+54 5 FT RT 12,900 12,875 25

BH20-04

Northbound

Shoulder

42+66 20 FT LT 12,865 12,835 30

BH20-05 Northbound 47+07 5 FT LT 12,846 12,821 25

BH20-06 Southbound 50+34 5 FT RT 12,836 12,806 30

BH20-07 Northbound 54+93 5 FT LT 12,836 12,806 30

BH20-08 Southbound 60+65 7 FT RT 12,844 12,824 20

BH20-10 Northbound 42+66 5 FT LT 12,865 12,845 20

Note: The exploration locations and elevations were estimated from Google Earth and in relation to posted mile markers.

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3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the borings by SPT and brass California liners were tested in the laboratory to support the field classifications and to provide an estimate of the engineering characteristics and mechanical properties of the soil. Laboratory tests included sieve analysis

(AASHTO T11 & T27), in-place density (ASTM D2937) classification (AASHTO M145 &

ASTM D2487), Atterberg limits (AASHTO T89 & T90), R-Value (AASHTO T190), resistivity

(AASHTO T288), and pH (AASHTO T289). When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. Results of the testing are shown below in Table 1.2 and are presented in

Appendix B.

Table 1.2:- Summary of Laboratory Index Test Results

BORING

NUMBER

SAMPLE

DEPTH, ft

PERCENT

GRAVEL

PERCENT

SAND

PERCENT

PASSING

#200

LL PI USCS AASHTO

BH20-02 1 - 5 15 71 14 NV NP SM A-1-b (0)

BH20-02 2.5 19 68 13 NV NP SM A-1-b (0)

BH20-02 10 27 53 20 22 3 SM A-1-b (0)

BH20-02 25 39 52.1 8.9 NV NP SW-SM A-1-a (0)

BH20-04 10 18 61 21 19 1 SM A-1-b (0)

BH20-05 2.5 – 5 36 56 8 NV NP SW-SM A-1-a (0)

BH20-05 15 14 59 27 23 3 SM A-2-4 (0)

BH20-05 25 16 60 24 26 7 SC-SM A-2-4 (0)

BH20-06 10 10 63 27 28 5 SM A-2-4 (0)

BH20-06 30 19 65 16 NV NP SM A-1-b (0)

BH20-07 1 - 5 22 64 14 NV NP SM A-1-b (0)

BH20-07 10 16 58 26 26 6 SC-SM A-2-4 (0)

BH20-07 25 10 45 45 55 25 SM A-7-5 (7)

BH20-08 2.5 9 70 21 NV NP SM A-1-b (0)

BH20-08 10 69 23.9 7.1 * * * *

BH20-08 15 0 63 27 * * * *

BH20-10 1 - 5 19 69 12 NV NP SW-SM A-1-b (0)

BH20-10 2.5 39 51.9 9.1 NV NP SW-SM/PT A-1-b (0)

BH20-10 2.5 0 94.9 5.1 NV NP SP-SM/PT A-1-b (0)

BH20-10 15 – 20 15 67 18 NV NP SM A-1-b (0)

*An insufficient amount of material was provided to perform AASHTO T89 & T90. Classification could not be determined.

Soil test results indicated silty sand soils with varying amounts of clay and both well-graded and poorly-graded sand content. Additionally, two shallow samples from BH20-10 included high organic content in their classification (PT). Soil samples were classified as A-1-a (0), A-1-b (0), A-2-4 (0), and A-7-5 (7) by AASHTO, and SM, SW-SM, SP-SM, SC-SM, and PT by USCS.

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R-Value (AASHTO T190), corrosivity of soils (AASHTO T288 & T289), and in-place density

(ASTM D 2937) tests were performed on select samples at the request of the CFLHD pavement engineer. Testing results are summarized below in Table 3.3.

Table 3.3:- Summary of Laboratory Engineering Property Test Results

BORING

NUMBER

SAMPLE

DEPTH, ft

RESISTIVITY,

ohm-cm

PH R-VALUE

IN-PLACE DENSITY

pcf

BH20-02 1 - 5 12,600 8.8 74 -

BH20-10 1 - 5 - - 73 -

BH20-10 2.5 - - - 61.8

BH20-10 15 – 20 - - - 151

3.3 SUMMARY OF SITE CONDITIONS

This section presents the results of the surface reconnaissance and subsurface exploration.

3.3.1 General Topography

Mount Blue Sky is the highest point in the Chicago Peaks massif, dominating the western skyline of the Great Plains, where it is visible from over 100 miles to the east. The project area is in a high alpine environment above tree line. The general topography of the area is largely characterized by mountainous terrain, alpine wetlands, and glacial features.

The project area is located within a cirque, an amphitheater-like valley formed by glacial erosion.

Typically, the floor of a cirque is bowl shaped due to glacial excavation, with three or more over-steepened headwalls and a lip or sill where the glacier flowed downhill out of the cirque. At least seven (7) glacial cirques cut into the Chicago Peaks massif, and the cirques around Mount Blue

Sky are some of the deepest in the Colorado Rockies (Bowman, 1914). Post-glacial lakes, referred to as tarns, are commonly found throughout the area. Tarns form in the hollow, bowl-shaped features left by cirques once the glacier has retreated. The more notable tarns in the area are

Summit Lake, the Chicago Lakes, Lincoln Lake, and Abyss Lake. Tarns may also be found in the surrounding U-shaped valleys.

The project alignment exists within the Summit Lake Park wetland complex. The Mount Evans

Wilderness area is generally classified as an alpine tundra but is unique in that small regions of arctic tundra exist around Summit Lake, supporting rare fauna that otherwise occur only above the

Page | 9 arctic circle. Unlike typical Colorado alpine tundra, characterized as dry once the snow recedes, arctic tundra holds small alpine pools of water which support diverse community types. Surface and subsurface flow paths within the Summit Lake Park wetland complex have been altered by

SH5, causing a hydrologic reorganization that has dewatered portions of the wetland and flooded others, impacting the rare and diverse vegetation (Shaw and Cooper, 2021).

3.3.2 Surface Reconnaissance

The existing pavement in the project area is generally in poor-condition with frequent transverse and longitudinal cracking, as well as dips and ridges. Portions of the roadway are lower than the surrounding ground and collect water. A site visit involving surface reconnaissance was conducted on September 1st & 2nd, 2020 by CFLHD Geotechnical Engineers to map geotechnical features and pavement distress along the project alignment. Notes and observations are attached as a KMZ file, and associated photos are attached as a ZIP archive. Select site condition photos are found in

Appendix C of this report.

Within the project area, eleven (11) cross culverts and one (1) parallel culvert were identified.

Culvert conditions varied between no adverse conditions, half-filled with sediment, buried outlets, and crushed due to embankment settlement. More information may be found in the final hydraulics report, dated July 17, 2023.

Colluvial deposits dominate the surface and contain particle sizes ranging from clay (< 7.9x10-5 inch) to boulders (>10.1 inch). Granitic boulders were typically several (2 to 5) feet in diameter, with some exceptions up to 15-feet in diameter. Sand, gravel, cobbles, and boulders were typically sub-rounded to sub-angular, and the fine material was typically non-plastic. Within the colluvial deposits exists pockets of “nested” boulders visible at the ground surface. Larger or oddly shaped boulders could be present in the subsurface. There is evidence that where surface drainage intersects with these “nests,” the surface water is lost and introduced into the subsurface.

3.3.3 Subsurface Conditions

SH5 is unique in that it is located above timberline in an alpine tundra environment. Permafrost is present in the subgrade. A portion of the subgrade above the permafrost thaws during the summer and freezes again during the winter. This is referred to as the active zone. The depth of the active zone varies naturally depending on altitude, climate, slope aspect, and vegetation cover. Areas of alpine permafrost are contracting due to disequilibrium with current climate conditions, and this decline will continue with ongoing global climate changes. Permafrost degradation begins with increasing seasonal thaw depths that eventually form progressively larger vertical taliks

(depressions or voids in permafrost) and continued melting thins the permafrost layer from both above and below (Shaw & Cooper, 2021).

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Furthermore, infrastructure development can affect the depth of the active zone by altering the thermal regime of the underlying soils. Paved roads can increase the depth of the active zone by decreasing the albedo (solar reflectivity) of the ground surface, causing more heat to be absorbed from the sunlight striking the road. This can create a “thaw-ribbon” immediately under and adjacent to the roadway. Deep thaw ribbons or taliks beneath embankments intercept and store groundwater, creating a feedback loop that increases thaw depth. Significant heat transfer and melting also occurs where water ponds against embankment toes and in roadside ditches, as well as from water flowing through culvert crossings (Shaw & Cooper, 2021).

Degradation of permafrost underneath SH5 is evidenced by the severe roadway damage. Ground-penetrating radar surveys performed as part of Colorado Department of Transportation (CDOT) funded hydrologic study of the area indicated 1.35- to 4.68-meter-thick active zones (4.43 to 15.35 feet) (Shaw & Cooper, 2021). The greatest depths to permafrost were found directly beneath the center of the roadway and the shallowest depths were west of the SH5 corridor. Additionally, the subsurface investigation in mid-September of 2020 revealed permafrost depths ranging from 15 to

30 feet throughout the project corridor. Thawing permafrost may also lead to a perched water table as water cannot percolate downwards through the frozen subsurface. This diverts subsurface flow beneath and along the roadway. The saturated subgrade soils may gradually move down slope

(solifluction), leading to further pavement damage.

The subsurface conditions along SH5 were investigated by advancing bore holes with ODEX drilling spaced across the project alignment. All borings displayed a silty sand material to termination depth (20 to 30 feet), with varying amounts of clay and gravel content. Boulders were encountered in the subsurface in borings BH20-02, BH20-04, BH20-05, BH20-07, and BH20-10.

Boulders encountered were up to approximately 2 feet in diameter. A summary of permafrost depths encountered during drilling are presented below in Table 3.4. Note that these permafrost depths were determined by identifying frozen water in samples collected from the borings, and therefore represent a maximum depth to permafrost. Permafrost conditions may be shallower and not identified correctly by this method where soil conditions are dry, where ice crystals were too small to be easily observed and identified, or where all frozen water is melted by drilling action prior to observation.

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Table 3.4:- Summary of Permafrost and Groundwater Depths Encountered During

Drilling

Boring Number

Permafrost Depth

Below Top of Boring

(ft)

Groundwater Depth

Below Top of Boring

(ft)

BH20-02 17 14

BH20-04 Not encountered Not encountered

BH20-05 25 11

BH20-06 25 6

BH20-07 24 5

BH20-08 14 10

BH20-10 15 5

3.3.4 Groundwater

Groundwater was encountered during drilling at depths ranging between 5 and 14 feet and are tabulated in Table 3.4 above. However, fluctuations in groundwater level due to seasonal and climatic effects are expected.

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the air convention embankment, general embankment, cut slopes, material shrink/swell, and construction considerations for the design and reconstruction of SH5. Based on discussions with the project team and through examining scientific literature, an air convection embankment is the preferred design solution to minimize permafrost degradation, provide positive drainage of surface and near surface water, and increase roadway stability. Consideration was also given to a dry bridge, sun/snow sheds, and subgrade insulation. These design measures were determined unfeasible due to effectiveness, cost, and visual impact to the public experience. Furthermore, items like sun/snow sheds and subgrade insulation would also not adequately address hydrologic restoration.

Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 AIR CONVECTION EMBANKMENT DESIGN

The project alignment on SH5 passes through wetland and fen areas and is underlain by silty sand containing permafrost. Current construction has yielded inadequate pavement performance, accelerated permafrost degradation, and disrupted both surface and subsurface water flow. Repair and replacement of damaged sections with conventional roadway and pavement design details will not remedy these problems, which are likely to increase with ongoing climate change (Shaw &

Cooper, 2021).

4.1.1 Design Selection

An air convection embankment (ACE) placed for the full width of the roadway embankment is proposed to stabilize the roadway, mitigate permafrost degradation, and to restore function in wetland areas. Air convection embankments consist of coarse, poorly-graded aggregate to provide high porosity (approximately 40%) and permit free air circulation (Shaw & Cooper, 2021). ACE allows convective air movement to occur during the cold winter months, cooling the underlying soils. During the winter, the surface and upper portion of the ACE cools due to the cold ambient air, while the base of the embankment remains comparatively warm. This creates an unstable air density gradient, whereas the cold air (higher density) sinks downward to be replaced by relatively warmer air (lower density) moving upward. This convection accelerates heat transfer from the base of the embankment to the surface at a rate more than an order of magnitude higher than conduction

(Doré et al, 2016). During the warm summer months, the upper portion of the ACE is warmed by solar radiation and warm air, while the base of the embankment remains cool. This leads to a stable air density gradient, the convective cell shuts down, and the embankment acts as an insulator, Page | 13 maintaining subsurface temperatures and reducing the depth of the active zone. In summary, the

ACE increases winter cooling rates and decreases summer warming rates.

Furthermore, the ACE allows surface water to diffuse through the embankment, rather than being captured in roadside ditches and discharged via culverts. By reducing the need for roadway drainage structures there is less risk to further altering alpine wetland hydrological and ecological functions. Drainage through the embankment also minimizes damage from frost heave and thaw weakening.

4.1.2 Thermal Modeling

A 2-D finite element model was developed using two modules from the GeoStudio software suite, AIR/W and TEMP/W, to evaluate the passive cooling capabilities of the ACE and its effects on the soil temperature profile and active zone depth.

Model Geometry & Mesh

The model geometry was developed using a combination of subsurface data from the drilling investigation and topographical data from a survey conducted by FHWA CFL. Multiple ACE geometries were modeled to determine final design, including: 4-foot, 5-foot, and 6-foot height embankments with side slope ratios of 1V:2H, 1V:4H, 1V:6H. A side slope ratio of 1V:6H was only modeled for the 4-foot-tall embankment, as the large construction footprint of a 1V:6H side slope for the 5-foot and 6-foot-tall ACE was anticipated to be unacceptable due to visual and environmental impacts. An example of the model geometry is shown in Figure 4.1 below.

Figure 4.1: Model geometry and material regions. Note: The model continues to 100-foot depth.

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The surficial topography of the model was developed for STA 34+00 as this was the approximate station of deepest permafrost found during Ground-Penetrating Radar (GPR) mapping in August

2020 by Colorado State University (Shaw & Cooper, 2021). The subsurface regions were simplified to be composed of a silty sand to depth. The modeled region is 90 feet wide and 100 feet deep with a 20-foot-wide asphalt roadway surface. The width was extended to 90 feet to eliminate possible edge effects, and the depth was extended to 100 feet to eliminate interference between the upper and lower boundary conditions.

Mesh element and type were varied throughout the model depending on region and depth. Four regions were assigned to the model: ACE (Region 1), 0 to 15-foot depth (Region 2), 15- to 30-foot depth (Region 3), and 30- to 100-foot depth (Region 4). The upper regions, 1 through 3, were assigned a mesh of quads and triangles with edge lengths ranging from 0.15 to 1.5 feet, increasing with depth. A rectangular grid of quads with edge lengths of 2 feet was assigned to region 4.

Greater element density was assigned near the surface of the model to obtain better resolution of the active zone. Figure 4.2 below depicts the model mesh used for all geometries.

Figure 4.2: Model mesh for embankment and foundation soils. Note: The model continues to 100-foot depth.

Boundary Conditions

Both thermal (TEMP/W) and air (AIR/W) boundary conditions were assigned to the model. The

ACE was modeled for two scenarios: 1) open interaction with the atmosphere, and 2) closed interaction with the atmosphere. The open surface allowed infiltration of surrounding ambient air directly into the ACE, simulating exposed side slopes in the summer and plowed snow conditions in the winter. The closed surface scenario prevented outside air from flowing into or out of the

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ACE, simulating snow covered conditions in the winter and topsoil covered side slopes in the summer. This results in an embankment core that is essentially cutoff from the surrounding air mass, allowing only internal convection to occur (Goering, 2003). The open surface scenario was simulated by assigning a barometric air pressure boundary to the side slopes of the ACE. The boundary was only assigned to the side slopes as the driving surface is to be paved and assumed impermeable. No air boundary condition was applied to the ACE in the closed surface scenario, simulating all surfaces to be impermeable.

The TEMP/W boundary conditions consisted of a heat flux boundary and multiple n-factor surface temperature boundary functions. A constant heat flux of 7.484675e-06 BTU/sec/ft2 was applied to the bottom boundary to simulate the average geothermal heat flux in the Front Range of Colorado

(Zacharakis, 1981). Three surface temperature boundary functions were assigned to simulate the natural ground, ACE surface, and asphalt. The surface temperature boundary functions used an n-factor approach, where n-factors (ratios of air thawing or freezing indices to the surface thawing or freezing indices) were applied to the model air temperature function. Two separate daily min/max sinusoidal functions were developed as the model air temperature functions. The first function was developed with historical climate data from a climate station located near Ward, CO at approximately 11,600 feet in elevation. Historical climate data from the Mount Evans Research

Station and Echo Lake Station were originally used in the analysis but were later deemed less representative of the project site than the Ward, CO data due to the climate station being located at an elevation approximately 2,500 feet below the project roadway (approximately 10,600 feet in elevation). Historical data from 01/01/1991 to 12/31/2021 was used for the spin-up analysis and estimated to be a significant enough duration to optimize the initial field of the model and improve its forecasting ability. The second daily min/max sinusoidal function was developed to allow for modeled projections to year 2050. This was done by averaging daily temperatures from 2012 to

2021 to use as baseline temperatures to then extrapolate out for the next 30 years. Air temperatures were increased by 0.07°F per year to simulate a projected 2.1°F mean temperature increase in

Colorado by 2050 due to global warming (Lukas et. al, 2014). Table 4.1 summarizes all n-factors used in the models. Initial ground surface and asphalt n-factor values were obtained from Darrow

(2011) and Darrow (2016), and then slightly adjusted until the model achieved an expected soil temperature profile and active zone depth.

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Table 4.1:- Summary of Model n-factors

Surface Freezing n-factor (nf) Thawing n-factor (nt)

Asphalt 0.70 1.80

Natural Ground 0.15 0.45

ACE Side Slope 0.30 1.00

Material Properties

For AIR/W analysis a single-phase model was used for all materials. Table 4.2 is a summary of all

AIR/W input parameters used for analysis.

Table 4.2:- Summary of AIR/W Parameters

Material Air Conductivity

(ft/sec)

Volumetric Air

Content

Activation Air

Pressure4 (psf)

Asphalt 1e-081 0.0013 0

Silty Sand 1e-081 0.0013 0

ACE 0.7591 0.32 0

1Darrow 2016 2Assumed porosity of ACE 3Arbitrarily low value to restrict airflow in non-ACE material 4Activation value required for transient analysis due to no preceding steady-state analysis

For TEMP/W analysis, both simplified and full thermal material models were utilized. The full thermal model was applied to the silty sand, requiring thermal conductivity and unfrozen water contents as a function of temperature. The thermal conductivity and unfrozen water content functions were approximated using TEMP/W’s internal estimation tool. The simplified thermal model was applied to the asphalt due to its negligible water content, assuming that phase change occurred at the phase change temperature and was not dependent on unfrozen water content.

Similarly, the simplified thermal material model was also applied to the ACE material as it is very well-drained with negligible water content. A negligible volumetric water content of 1% was used for the model to solve the equations, resulting in no change in thermal conductivity or volumetric

Page | 17 heat capacity for the frozen and unfrozen states (Darrow, 2016). Additional input parameters and sources are summarized in Table 4.3.

Table 4.3:- Summary of TEMP/W Parameters

Material Kf

(BTU/sec/ft/°F)

Ku

(BTU/sec/ft/°F)

Cf

(BTU/ft3/°F)

Cu

(BTU/ft3/°F)

Θ4

Activation

Temperature3

(°F)

Asphalt 0.0001863011 0.0001863011 34.019081 34.019081 0.11 33

Silty

Sand

0.0003148152 0.0001597222 47.752 29.82 40.72 28

ACE .0000474542 .0000474542 19.72 19.72 12 28

1Darrow 2011 2Darrow 2016 3Activation value required for transient analysis due to no preceding steady-state analysis.

4Kf and Ku are frozen and unfrozen thermal conductivity, respectively, Cf and Cu are frozen and unfrozen volumetric heat capacity, respectively, and Θ is volumetric water content.

Modelling Stages, Durations and Time Steps

The first stage of modeling involved developing the initial ground temperatures and air flow distribution for the site. This was done by running a transient model from 01/01/1991 to

12/31/2021 (11,323 days) with a 1-day time step. For this spin-up analysis, heat and air transfer were run uncoupled (in parallel) as very little to no natural convection was expected to occur in the silty sand due to its predominantly fine-grained composition (Farouki, 1981). This stage was modeled multiple times while adjusting n-factors until a reasonable ground temperature profile and depth to permafrost was achieved, based on the previously mentioned GPR data. The final temperature and air flow results were used as inputs for the proceeding analyses.

The second and final stage of modeling involved multiple analyses, including one with no roadway improvements and multiple with various ACE geometries, projected to 12/31/2050 (10,592 days) with a 1-day time step. For these analyses, heat and air transfer were coupled to force convection with air transfer, so that heat energy moves within the domain because of both conduction and advective processes. The multi-physics formulation uses the air fluxes to compute and assemble the advective heat (energy) transfer terms into the global finite element equation. An analysis was run with no improvements to the roadway to serve as a baseline for comparison to better determine the improvements an ACE will provide to the subsurface temperature profile. Details of modeled

ACE geometries was discussed in the Model Geometry & Mesh subsection above. The results of each month in the final year were saved and exported for further analysis.

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Results & Discussion

The spin-up analysis modeled depth to permafrost as approximately 11.5 feet beneath centerline and approximately 7 feet beneath roadway shoulders (Figure 4.3). Permafrost depth was estimated to a mean depth of approximately 12.3 feet beneath centerline and 6.6 feet beneath roadway shoulders, using GPR data collected by Shaw & Cooper (2021). The modeled depth to permafrost beneath centerline and shoulders in the spin-up analysis was within +/- 6 inches to the interpreted subsurface conditions collected by the GPR survey.

Figure 4.3: Results of spin-up analysis , 12/31/2021. The dashed red line indicates the 32°F isotherm.

Following the spin-up analysis, a 2050 projected model with no ACE (accounting for global warming) was analyzed. Results of this analysis indicated an increase in permafrost depth from

11.5 to approximately 18 feet (Figure 4.4). Temperature contours within the subsurface increased in thickness, indicating a general warming of the soils with depth. Furthermore, the 32°F isotherm became deeper with a more pronounced dip compared to the 2020 model, indicating continued impacts on permafrost depth due to the roadway surface.

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Figure 4.4: Results of no ACE construction projected to 12/31/2050. The dashed red line indicates the 32°F isotherm.

Next, as discussed in the Model Geometry & Mesh subsection, a range of ACE heights and side slope ratios were modeled to 2050 using input parameters from the spin-up analysis. For each embankment geometry results were saved every year from 2020 to 2050 at the end of July, and the last day of the analysis, 12/31/2050.

In the initial iterations of the model, side slopes of 1V:2H, 1V:4H, and 1V:6H were analyzed for the 4-foot ACE. Preliminary results indicated that gentler ACE side slopes did not have a noticeable effect on the subsurface temperature profile. Effects of the side slope angle were modeled to be neutral or negative. For instance, the 1V:6H slopes pushed the 32°F isotherm deeper into the subsurface in comparison to the 1V:2H slopes, resulting in a negative effect due to warmer temperatures near the ground surface (Figure 4.5 and 4.6). When comparing 1V:2H to 1V:4H slopes, changes to the temperature profile are indiscernible. Figures 4.5 and 4.6 come from the first iteration of analysis using the climate data from a lower elevation, as discussed in the

Boundary Conditions subsection. Due to this observation, only 1V:2H side slopes were modeled in the second round of analysis with climate data from Ward, CO. Steeper side slopes were selected for final analysis due to the negligible effects the slope ratio has on modeled ACE efficacy, and for environmental and aesthetic considerations to reduce the overall construction footprint of the embankment.

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Figure 4.5: 4-foot ACE with 1V:2H side slopes, 12/31/2050. The dashed red line indicates the 32°F isotherm and blue arrows indicate air velocity vectors. Output from preliminary analysis.

Figure 4.6: 4-foot ACE with 1V:6H side slopes. The dashed red line indicates the 32°F isotherm and blue arrows indicate air velocity vectors. Note development of 44-46°F temperature contour. Output from preliminary analysis.

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Final results indicate development of convection eddies within ACE side slopes and minor air movement within the core of the embankment (Figure 4.7 & 4.8). The cooling effects were modeled to be greater in the 6-foot ACE (Figure 4.8) due to larger and more pronounced convection eddies. The development of larger eddies in the 6-foot ACE effectively pulled more heat out of the soil, as evidenced by the temperature contours being pulled higher into the embankment. The greater passive cooling abilities of the 6-foot ACE in comparison to the 4-foot

ACE is depicted in Figures 4.9 & 4.10. The depth to permafrost was modeled to 18 and 15 feet on

December 31, 2050 for the 4-foot and 6-foot embankments, respectively. Additionally, the 32°F isotherm in both ACE scenarios is more flattened in shape, displaying a less pronounced dip. This effect is interpreted to display the effectiveness of the ACE in mitigating the impact of the roadway on permafrost. For comparison, a very pronounced dip in the isotherm is visible in Figure 4.3, and also evidenced by the large difference in depth to permafrost beneath the centerline and shoulder gathered from the GPR survey.

Figure 4.7: 4-foot ACE with 1V:2H side slopes, 12/31/2050. The dashed red line indicates the 32°F isotherm (note a deeper isotherm exists as well in Figure 4.9) and blue arrows indicate air velocity vectors.

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Figure 4.8: 6-foot ACE with 1V:2H side slopes, 12/31/2050. The dashed red line indicates the 32°F isotherm (note a deeper isotherm exists as well in Figure 4.10) and blue arrows indicate air velocity vectors.

Figure 4.9: 4-foot ACE with 1V:2H side slopes, 12/31/2050. The dashed red line indicates the 32°F isotherm and blue arrows indicate air velocity vectors.

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Figure 4.10: 6-foot ACE with 1V:2H side slopes, 12/31/2050. The dashed red line indicates the 32°F isotherm and blue arrows indicate air velocity vectors.

Results of the 4-foot and 6-foot ACE modeling is summarized and presented in Table 4.4 and

Figure 4.11 below.

Table 4.4:- Air Convection Embankment Model Results

Model

Scenario

Height Slope Ratio Depth of 32°F Isotherm

Beneath Centerline

(FT) (V:H) (FT)

No ACE - - 18

ACE

4 1:2 18

6 1:2 15

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Figure 4.11:- Modeled permafrost depths from construction to 2050.

While Table 4.4 indicates no overall effect from the 4-foot ACE and a 3-foot reduction in depth from the 6-foot ACE by 2050, Figure 4.11 provides additional insight into the efficacy of the ACE structures. The trends presented in Figure 4.11 indicate positive impacts of the 6-foot ACE compared to no-ACE until 2050, and positive impacts of the 4-foot ACE until approximately 2044, and neutral to minor positive impacts following that year. Additionally, as previously discussed in this subsection, the 32°F isotherm shows a flattening beneath the roadway, indicating a mitigation of the roadways impact on subsurface soil temperatures. The flattening of the 32°F isotherm, while still increasing to the final or near final depth of the isotherm modeled in the no ACE scenario, in combination with the reduced efficacy of the ACE structure over time, is interpreted to indicate that while the ACE is able to mitigate roadway impacts on permafrost depth in the next few decades, it will be unable to outpace the effects of global warming into the midcentury.

Finally, while the mitigative effects of the ACE on permafrost depth diminish over time, the ACE will continue to effectively reduce or eliminate roadway settlement and help restore the currently interrupted hydrologic regime well into the late century.

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4.1.3 Air Convection Embankment Recommendations

Summary of ACE design

The ACE is recommended to be constructed at a 4-foot height with 1V:2H side slopes. Class 1

Type C nonwoven geotextile fabric should be placed below the base or subbase course and above the ACE aggregate material to prevent pores within the ACE from filling with fines. Geotextile fabric should also be placed between the ACE and the natural ground or embankment subgrade.

Aggregate used for ACE construction should range in size between 4 to 12 inches and have a relatively uniform size.

Discussion

Embankment geometry is a key component to increasing heat extraction and reducing heat gain.

Raising embankment height may increase thermal resistance, and gentler side slopes (1V:4H to

1V:6H) may reduce accumulation of snow drifts and cause water to drain further away from the toes. By reducing snow cover on the embankment slopes, this facilitates greater heat loss during winter, helping to raise the active zone, while also reducing ponding of water against the toe, thus lowering heat advection from water (Shaw & Cooper, 2021).

Modeling did not show improvements to the active zone from reducing the side slope angle, which may be attributed to the assigned boundary conditions not adequately representing a scenario of snow accumulation. Inclusion of a land-climate interaction boundary could be considered for future modeling to account for rainfall, evapotranspiration, snow melt, and run off, but it is unlikely these fluxes will have much influence on the thermal response of the system. Additionally, gentler side slopes increase the construction footprint and raised environmental and visual impact concerns. Finally, while modeling greater embankment heights did display larger improvements to the thermal profile in the underlying soils by 2050, a 4-foot ACE was still shown to have mitigative effects in the next few decades, and a taller ACE would also further contribute to an increased construction footprint and costs. With this in mind, the 4-foot ace with 1V:2H side slopes was selected to be incorporated into design.

Albedo of aggregate increases with particle diameter for sizes larger than 5 centimeters (2 inches), and structures built with large aggregate experience lower heat loading (Zhang et al., 2019).

Whereas smaller diameter aggregate increases effective albedo (Qin et al., 2016, Zhang et al.

2019). Scientific literature cited effective aggregate size ranges of: 4 to 8 inches (Xu & Goering, 2008), 8 to 12 inches (Cheng et al., 2009), and 4 to 12 inches (Doré et al., 2016). A range of 4 to

12 inches was selected for design as a specification for a tighter aggregate size range may be more difficult and costly for the contractor to process and produce.

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4.2 EARTHWORKS

4.2.1 Shrink/Swell Recommendations

On-site soils are glacial deposits consisting of silty sands with varying amounts of clay and gravel.

It is estimated these soils will have a 10% shrink percentage, corresponding to a shrink/swell factor of 0.90. The recommended shrink/swell factor is based on standard tabled values for common materials in the FLH Technical Guidance Manual (2006).

4.2.2 Cut Slopes

Cut slopes are anticipated during construction of the proposed ACE. In general, existing slopes along the project corridor are gentle (1V:4H) to flat, but limited sections of 1V:1H to 1V:3H slopes are present. Slopes are recommended to be cut no steeper than 1V:2H in the absence of seepage.

If seepage is encountered during construction, slopes should be laid back to a stable angle.

Furthermore, ground disturbance should be minimized to the greatest extent possible and is discussed further in the Error! Reference source not found. section below.

4.2.3 Embankments

Refer to the above subsection titled Air Convection Embankment for design and construction details.

Additionally, fill material should be placed beneath the ACE section and graded 2.0% downhill

(minimum), or as recommended by the hydraulics engineer, to ensure positive drainage of surface and near surface water.

All embankment work should be placed and compacted in accordance with the requirements of

Section 204 of the FP-14.

4.2.4 Granular Section

A special granular section is recommended and anticipated to be excavated and constructed from

STA 52+17 to 61+50. A granular section is proposed in this area, rather than an ACE, as the existing road conditions are less severe in comparison to other segments of the road, and to limit construction impacts. The granular section will excavate and replace thaw sensitive material immediately below the pavement section to prevent frost heave and thaw settlement. The granular section is to be constructed to a minimum 2-foot depth, lined with a geotextile, and filled with granular backfill with a stabilization geogrid on top beneath the structural section. See the Granular

Typical Section sheet within the project plans for more details.

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4.3 CONSTRUCTION CONSIDERATIONS

Excavation: Excavate using equipment capable of removing the material while preventing material from moving outside the construction limits.

Boulders are expected to be found within the subsurface.